One of the oldest problems of communications engineering is how to efficiently produce double-sideband amplitude modulated waves and other envelope modulated waves Numerous amplitude modulation systems have been proposed and a large variety of such systems are in current use, including plate or collector modulation, forms of pulse width modulation, Doherty modulation, and Chireix modulation.
Chireix modulation, although a very modern appearing system, was originally disclosed by its French inventor in the early 1930s. The Chireix system, shown in block form in FIG. 1, is discussed in more detail below where improvements on this ingenious system are disclosed.
It is noteworthy that an early publication describing the Chireix system ("High Power Outphasing Modulation", H. Chireix, November 1935, PIRE, pgs. 1370 to 1392) discussed the use of another basic invention. That basic invention was a method for producing phase modulated waves combining an unmodulated wave (carrier wave) and an ". . . almost entirely modulated wave"; i.e., wherein the carrier is almost entirely suppressed such as produced by a balanced modulator, said almost entirely modulated wave is 90 degrees out-of-phase with the unmodulated wave. This technique was also used by E. H. Armstrong in his early FM experiments and is generally called the Armstrong modulator. The basic Armstrong modulator is shown in FIG. 2.
With the advent of transistors, and the common usage of very high frequencies, the need for new improved high efficiency systems for producing envelope modulated waves has become more pressing. The efficiency of a conventional high frequency transistorized linear amplifier, above 800 MHz, is very poor. Many commercial linear amplifiers for operation at 800 to 1,000 MHz have efficiencies of significantly less than 10%.
Attempts have been made to provide an improved linear amplifier (D. C. Cox, "Linear Amplification with Nonlinear Components", IEEE Transactions on Communications, Vol. COM-22, pp. 1942-1945, December 1974) by combining the Chireix system with the Envelope Elimination and Restoration (EER) system, see L. R. Kahn, "Single-Sideband Transmission by Envelope Elimination and Restoration", PIRE, Vol. 40, No. 7, July 1952, pgs. 803-806 and L. R. Kahn, "Comparison of Linear Single-Sideband Transmitters with Envelope Elimination and Restoration Single-Sideband Transmitters", PIRE, Vol. 44, No. 12, December 1956, pgs. 1706-1712. Mr. Cox remarked in the first sentence of his December 1974 publication:
"Conventional solid-state linear power amplifiers are difficult to build at low microwave frequencies and impossible to build at high microwave and millimeter wave frequencies." PA1 (a) generating an unmodulated carrier wave, PA1 (b) feeding said unmodulated carrier wave to one of the three signal paths, PA1 (c) phase modulating a portion of the carrier wave generated in step (a) carrier wave to produce a first phase modulated wave, PA1 (d) feeding said first phase modulated wave to a second signal path, PA1 (e) phase modulating another portion of the carrier wave generated in step (a) to produce a second phase modulated wave, PA1 (f) feeding said second phase modulated wave to the third signal path, and, PA1 (g) combining the output signals of the three paths to produce the desired envelope modulated wave.
A tutorial paper that discusses approaches to high efficiency envelope modulation systems is, "High Efficiency Amplification Techniques", Dr. F. H. Raab, IEEE Circuits & Systems (Newsletter), 7, pp. 3-11, December 1975. This paper includes treatments of the Chireix systems, and the EER system, as well as a number of other high efficiency systems.
Finally, an excellent treatment of the Chireix system, including suitable combining networks, is contained in the book Radio Engineering, E. K. Sandeman, Chapman & Hall Ltd., London, Second Edition, 1953, Chapter XIV.